This invention relates to electrostatographic copying and more particularly to a novel electrostatographic photosensitive device. The art of xerography, as originally disclosed in U.S. Pat. No. 2,297,691 by C. F. Carlson, involves the formation of an electrostatic latent image on the surface of a photosensitive plate normally referred to as the photoreceptor. The photoreceptor comprises a conductive substrate having on its surface a layer of a photoconductive insulating material. Normally, there is a thin barrier layer between the substrate and the photoconductive layer to prevent charge injection from the substrate into the photoconductive layer upon charging of the plate's surface.
In operation, the plate is charged in the dark, such as by exposing it to a cloud of corona ions, and imaged by exposing it to a light shadow image to selectively discharge the photoreceptor and leave a latent image corresponding to the shadow areas. The latent electrostatic image is developed by contacting the plate's surface with an electroscopic marking material known as toner which will adhere to the latent image due to electrostatic attraction. Transfer of the toner image to a transfer member such as paper with subsequent fusing of the toner into the paper provides a permanent copy.
One type of electrostatographic photoreceptor comprises a conductive substrate having a layer of photoconductive material on its surface which is overcoated with a layer of an insulating organic resin. Various methods of imaging this type of photoreceptor are disclosed by Mark in his article appearing in Photographic Science and Engineering, Vol. 18, No. 3, pgs. 254-261, May/June 1974. The processes referred to by Mark as the Katsuragawa and Canon processes can basically be divided into four steps. The first is to charge the insulating overcoating. This is normally accomplished by exposing it to d.c. corona of a polarity opposite to that of the majority charge carrier. When applying a positive charge to the surface of the insulating layer, as in the case where an n-type photoconductor is employed, a negative charge is induced in the conductive substrate, injected into the photoconductor and transported to and trapped at the insulating layer-photoconductive layer interface resulting in an initial potential being solely across the insulating layer. The charged plate is then exposed to a light and shadow pattern while simultaneously applying to its surface an electronic field of either alternating current (Canon) or direct current of polarity opposite that of the initial electrostatic charge (Katsuragawa). The plate is then uniformly exposed to activating radiation to produce a developable image with potential across the insulating overcoating and simultaneously reduce the potential across the photoconductive layer to zero. In other processes described in the Mark article, i.e. the Hall and Butterfield processes, the polarity of the initial voltage is the same sign as the majority charge carrier and reverse polarity is encountered during erase.
In processes where the voltages must initially be placed across the overcoating, for example, in step 1 of the Canon process, either an injecting contact for the majority carrier or the ability to bulk generate carriers or an ambipolar photoconducting layer must be used. In processes where the initial voltage polarity is the opposite sign of the majority carrier, there is required an injecting contact for the majority carrier, the ability to bulk generate carriers or an ambipolar photoconducting layer.
It is an object of the present invention to provide a novel electrostatographic photosensitive device having a layer of an insulating organic resin on its surface.
A further object is to provide such a device which has mechanical flexibility and can be easily fabricated at a moderate cost.
An additional object is to provide such a device which provides mechanical, chemical and electrical protection for the electrically active components.
Another object is to provide such a device with improved dark injection efficiency.